1. Field of the Art
The present invention relates to an apparatus for detecting pressure pulse wave produced from an arterial vessel of a living body, at a plurality of positions on a body surface right above the arterial vessel.
2. Related Art Statement
Pressure pulse wave produced from an artery of a living body in synchronization with pulsation of the heart, provides information on the cardiovascular system of the subject such as active condition of the heart, blood pressure and degree of arterial sclerosis. There is known a device which detects the pressure pulse wave produced from an artery of a subject, by using a pressure sensor which is pressed against the artery via the body surface right thereabove. However, the pressure sensor is adapted to contact at a pressing surface thereof the body surface over a whole width (or diameter) of the artery. In other words, the pressing surface of the pressure sensor is larger than the width of the artery. For this reason the detection of the pressure pulse wave may be adversely influenced by the wall of the artery, and the detected pressure pulse wave may not provide accurate information.
To overcome the above-indicated problem it has been proposed to detect pressure pulse wave at a plurality of positions which are spaced apart from each other on the body surface in a direction crossing the artery, by a length smaller than the width of the artery. An example of the pressure-pulse-wave detecting device of this type is disclosed by U.S. Pat. No. 4,423,738. The device disclosed therein is capable of selecting the single one of the plurality of positions which is substantially free from the problem of the wall of the artery and at which a blood pressure approximate to an intra-arterial blood pressure is measured.
Further, the assignee of the present application has filed Japanese Patent Application No. 63-87720 on Apr. 9, 1988, in which it is proposed to measure a degree of arterial sclerosis of an artery of a subject by detecting pressure pulse wave at a plurality of positions on the body surface right above the artery. Japanese Patent Application No. 63-87720 was published under Publication No. 1-359836 on Oct. 17, 1989.
In the above-indicated devices in which pressure pulse wave is detected at a plurality of positions on the body surface right above an artery, however, the number of the detecting positions was insufficiently low, in particular in the case of an artery of a comparatively small width. More specifically described, the pressure sensor employed in those devices consists of a semiconductor plate of a comparatively large thickness and a comparatively high rigidity. The semiconductor plate includes a plurality of pressure-receiving portions (i.e., pressure-detecting elements) of a comparatively small thickness which are provided by forming a plurality of recesses in one of opposite surfaces of the plate in a direction of thickness thereof by photoetching technique. Upon application of a pressure to the semiconductor plate, a strain is produced in the pressure-receiving portions. However, if each pair of adjacent recesses are formed excessively near to each other, it is not permitted to form an electrically conductive wiring pattern in an area between the each pair of recesses. Further, in such a case the rigidity of the semiconductor plate as a whole is disadvantageously lowered, and the accuracy of pressure measurement is sacrificed. Thus, the pressure-receiving portions of the semiconductor plate must be spaced apart from each other by a distance greater than a required minimum length.